Northside Health Library

Klinefelter syndrome

Definition

Klinefelter syndrome is the presence of an extra X chromosome in a male.

Alternative Names

47 X-X-Y syndrome

Causes

Humans have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. The two sex chromosomes determine if you become a boy or a girl. Females normally have two XX chromosomes. Males normally have an X and a Y chromosome.

Klinefelter syndrome is one of a group of sex chromosome problems. It results in males who have at least one extra X chromosome. Usually, this occurs due to one extra X. This would be written as XXY.

Klinefelter syndrome occurs in about 1 out of 500 - 1,000 newborn boys. Women who get pregnant after age 35 are slightly more likely to have a boy with this syndrome than younger women.